Ya it's legit and pretty rare. A rundown on its history:
"These carts were offered as replacements for the original black cart when a major glitch was found in the game that made progressing past a certain set of levels in co-op mode impossible. Those who purchased the original game and then requested a fixed version by mailing in their black cart were sent one of these grey carts (there is thin evidence to support that the variant may have hit store shelves later as well, albiet extremely limitedly if so). This NTSC grey variant was almost unknown about until February 2012, when seller gamesgalore801 listed a box of 120 of the carts on Ebay. The games within the Nintendo-sealed box were loose, each inside their own plastic sleeve. Before this, little of these carts were seen for sale online and most collectors didn't agknowledge their existance. The total number of these made/released is unknown but considering that the game wasn't hugely popular, and that the original glitch didn't affect everyone (meaning less replacement requests), it's likely the one of the rarest NTSC N64 variants."
They don't appear for sale on ebay very often but since the 120 copies were all sold there are a lot in collector's hands atm. Just put it up for sale and see what you're offered.